1. Field of the Invention
This invention relates to the internal inspection of pipelines for carrying fluids, such as oil or gas, over considerable distance, subsea or overland, between accessible ends of individual pipeline sections. Pipelines of this kind which are normally inaccessible from the outside must be inspected at regular intervals from the inside for possible corrosion attacks, cracking, or other defects, in order to prevent costly production interruptions, catastrophic failure, and/or disasterous severances of the supply of energy to the community.
2. Background of the Prior Art
For such internal inspections, a great variety of apparatus carriers, pressure-driven "pigs" or self-propelled "crawlers," have been developed and used for carrying visual, magnetic, electric, ultrasonic or similar non-destructive examination apparatus through a pipeline from a launching trap at one end to a receiving trap at the other end of a pipeline section. Typically, such apparatus carriers may contain pressure-tight capsules for non-destructive examination apparatus, position determining apparatus, recording means, driving mechanisms, and battery or turbo-generator units for energizing the various apparatus in the carrier. After the passage of the apparatus carrier, the recording means may then be removed and connected to display and record means situated outside the pipeline for analysis, evaluation, and permanent recording of the results of the inspection. An extensive description of an inspection system of this kind has been given in Scherbatskoy U.S. Pat. No. 2,940,302.
It is a severe drawback of most known inspection systems of this kind that it is impossible to communicate with the apparatus carrier during its passage through the pipeline for obtaining instant on-line displays of the results of the inspection, and for controlling the functions of the non-destructive examination apparatus and the rates of progression of the apparatus carrier.
For relatively short pipeline sections, such as off-shore platform oil or gas risers, it is possible to use cable connected apparatus carriers communicating on-line with outside display, recording and control means. This requires, however, the very costly operations of interrupting production and emptying the pipeline section in question. For pipeline sections of more than a few hundred meters in length, the use and recovery of electric cables becomes impractical. An example of a system of this kind, using a cable connected apparatus carrier containing television and leak testing apparatus has been described in Cramer U.S. Pat. No. 3,400,574.
It has been proposed in the above-mentioned Scherbatskoy U.S. Pat. No. 2,940,302 to use the pipeline as a dielectric wave guide for the wireless transmission of acoustic or microwave signals between the apparatus carrier and an outside recording and control station. This system has, however, been found inapplicable due to antenna and transmission problems, and it has not found use in actual practice.